1. Field of the Invention
The present invention relates to a torque distribution control device for controlling the distribution of a drive torque to sub-drive wheels of a four-wheel drive vehicle.
2. Discussion of the Related Art
A torque distribution control device for a four-wheel drive vehicle has been known as described in Japanese unexamined, published patent application No. 2001-71781, wherein an electromagnetic clutch is provided for distributing the torque transmitted from an engine to prime drive wheels, to sub-drive wheels in dependence on a rotational speed difference between the prime drive wheels and the sub-drive wheels. In the known control device, when the acceleration of the vehicle exceeds a predetermined value, there is selected an acceleration mode map which has been set to enlarge the rate of the increase in the command torque distributed to the sub-drive wheels to the increase in the rotational speed difference between the prime drive wheels and the sub-drive wheels. As a consequence, when the rotational speed difference is large, a large electric current is applied to an exciting coil of the electromagnetic clutch, whereby the same transmits a large command torque to the sub-drive wheels.
However, in the known control method, even when the engine torque remains small, it often occurs that in dependence on the driving manipulation or the traveling state, a large command torque is calculated thereby to cause a large electric current to flow through the exciting coil of the electromagnetic clutch. This causes a control device (ECU) to generate heat, which in turn causes circuit elements such as resistances, condensers and so on to vary their characteristics. As a result, the accuracy in controlling the electric current to be applied to the exciting coil is lowered, so that it becomes impossible to precisely perform the torque distribution to the sub-drive wheels. For measures against this drawback, it becomes necessary to use those circuit elements which have small temperature variations of their characteristics, thereby resulting in disadvantage in respect of increasing the manufacturing cost.